Precipitation

Nb also has an influence on the hardenability of steel, the incubation time of allotriomorphic ferrite is longer in niobium-containing steels, soluble Nb segregates to the prior austenite grain boundaries and reduces the grain boundary energy [40], this mechanism improves the hardenability and compensates the low carbon concentrations found in MAS. The higher the Nb concentration in solid solution, the lower the critical temperature Ar3, low temperature transformation products are achievable when high Nb concentrations and fast cooling rates are combined.

2.2.1.2. Precipitation

Niobium belongs to the group of transition metals which have the characteristic to possess unfilled inner shells inside the outer or valence shell. Compared to non-transition metals, transition metals can dissolve much more readily small atoms such as nitrogen and carbon to form interstitial solid solutions [24]. Nb is a strong carbides and nitrides former but shows poor tendencies to form oxides and sulphides, this characteristic distinguishes it from Titanium which forms carbides until all the available oxygen, sulphur and nitrogen are consumed by initial additions of titanium [35] or other elements. Ti can form large non-metallic inclusions with oxygen and sulphur (particles which are in most of the cases negative for the mechanical properties), while Nb is confined to form much smaller precipitates which somehow or other contribute to the strengthening of steel.

The precipitation of Nb in steel results in three main compounds with a B1 (NaCl) type fcc crystal structure, their stoichiometry can be represented as NbCx, NbNx and NbCxNy. In the first two systems NbCx and NbNx x is equal to the C/Nb and N/Nb mole ratios respectively, whereas in NbCxNy x is the C/Nb ratio, y is the N/Nb ratio, and 1 – (x + y) is the mole ratio of vacancies. This type of precipitates are of most interest in Nb microalloyed steels, however, it is important to mention that other compounds can be found in some steels, such is the case of the precipitate Nb2C that has an hcp (hexagonal closed packed) crystal structure, this compound has been observed in steels with high Nb/C ratios [35].

The lattice parameter of the compounds varies according to their stoichiometry, for instance, the carbon solubility in NbCx reported at 1100°C falls in the range 0.72 < x < 1, since the lattice

23 parameter increases with x, the pure stoichiometric NbC has a lattice parameter of 4.470 Å while in NbC0.7 the lattice parameter is about 4.43 Å [35], [41]. The NbN shares very similar characteristics with NbC, in fact, these two compounds can easily combine to form carbonitrides type NbCxNy. The composition of the carbonitrides varies with the chemical composition of the steel, the N/C ratio plays a primary roll, the larger the ratio the higher the content of nitrogen in the precipitates. Other nitrides formers such as Ti and Al affect the composition of the carbonitrides. (hereinafter, NbCxNy will be denoted as Nb(CN) for simplification).

The precipitation of niobium occurs on preferential crystallographic directions of the matrix, thus certain orientations relationships between the matrix and the precipitates are conserved.

Due to the NaCl type crystal structure of the Nb precipitates, the precipitation in the parent austenite follows the next relationships [35]:

[100]M(CN) ǁ [100]γ

[010]M(CN) ǁ [010]γ

whereas in ferrite or martensite, the NbCxNy precipitation obeys the Baker-Nutting orientation relationship [42].

[100]NbC ǁ [100]α

[011]NbC ǁ [010]α

These orientation relationships (OR) are useful to determine whether the Nb precipitation happened in austenite or in its further transformations, i.e. ferrite or martensite. When the austenite transforms on cooling, it does so with the Kurdjumov-Sachs (K-S) OR [43].

(111)γ ǁ (110)α

[110]γ ǁ [111]α

Therefore, the origin of the precipitation can be traced back by analysing the crystallographic orientation relationships between the matrix and the precipitates at room temperature, if the ferrite and the precipitates are related by the K-S OR, the precipitates were formed in austenite,

24 otherwise, if they obey Baker-Nutting OR, it means that the precipitation occurred during or after the transformation from austenite to ferrite. If none of these orientation relationships between the matrix and the particles is accomplished, then the precipitation happened in austenite and further recrystallisation erased the crystallographic correlations between the matrix and the precipitates. It is important to notice that recrystallisation of austenite is not very frequent the after the Nb(CN) precipitation, most of the precipitates formed in the parent austenite are strain induced and have a strong effect on the retardation of the recrystallisation.

The precipitation of Nb(CN) on the steel matrix generates a significant lattice mismatch as shown in Table 2. The misfit strains around the particles strengthen the matrix and provide precipitation hardening, besides, the lack of registry is large enough that the coherency between the particles and the matrix within the grains is not possible, that is the reason why the precipitates nucleate only in the grain or subgrain boundaries and other crystalline defects.

Table 2. Lattice mismatch for Nb(CN) precipitates in austenite and ferrite [35].

The precipitation of Nb(CN) shows three typical distributions and morphologies: i) general precipitation, ii) interphase precipitation and iii) strain induced precipitation. The general and interphase precipitation occurs in ferrite when the remaining soluble Nb precipitates during or after the austenite transformation. The solubility of niobium in ferrite is around 20 times lower than in austenite, thus, a sudden supersaturation of soluble Niobium and a significant increase in the driving force for precipitation is expected when the γ to α transformation occurs. As for

25 the strain induced precipitation in austenite, the Nb(CN) precipitates are formed dynamically during the deformation, this type of precipitation is of major interest in thermomechanical controlled processing due to the austenite conditioning provided by the fine precipitated particles.

The general or homogeneous precipitation of Nb in ferrite occurs at the lowest temperatures during hot strip coiling or tempering of a quenched steel. The precipitates are very fine and homogenously dispersed in the matrix, after prolonged holding at high temperatures the precipitates remain fine and do not show a dramatic coarsening [44]. These precipitates are responsible for the precipitation hardening effect of Nb(CN) [35]. An example of the homogeneous precipitation of NbC in a microalloyed steel is shown in Figure 12.

Figure 12. Thin foil bright field TEM images showing homogenous precipitation of NbC in a steel containing 0.06C-0.056Nb (wt%) [44].

Interphase precipitation on the other hand, is observed at higher temperatures and lower cooling rates, the advance of the transformation front must be slow enough to allow for the nucleation of precipitates at the α-γ interphase. As the phase boundary moves to a new location, fine and spherical NbC precipitates are left behind in parallel rows as can be seen in Figure 13, the spacing among the rows depends on the temperature. Similarly, to general precipitation, the rows of fine precipitates strengthen the ferrite matrix.

26 Figure 13. STEM Bright field image showing the interphase precipitation of NbC at a) 750°C, b) 700°C and c) 650°C in a steel containing 0.052C-0.05Nb (%wt ) [45].

As previously mentioned the solubility of Nb in austenite is significantly higher than in ferrite, in fact, in undeformed austenite the precipitation of Nb is a very sluggish process which is a disadvantage for the hot rolling process, since the benefits of Nb as precipitate can be exploited only when the precipitation occurs sufficiently rapid. However, the kinetics of precipitation can be dramatically affected by deformation, the massive introduction of dislocations in the austenite as the strain increases provides nucleation sites to bring about the SIP of Nb. The morphology and distribution of SIP precipitates are optimal for the austenite refining since the particles are very small and dispersed, the size of precipitates has been measured in multiple studies and although there are some differences in the sizes reported, SIP precipitates observed just after the deformation are very fine and their average diameter ranges from a few nanometres to around 25 nm [46]–[50].

A proper combination of multiple deformation passes, strains, interpass times, and deformation temperatures can be designed for microalloyed steels to obtain the maximum strain induced precipitation of niobium, as the strain accumulates during the deformation passes, the volume fraction of precipitates increases and the austenite is shaped into an elongated body in the rolling direction with negligible recrystallisation, this is the principle of “austenite conditioning” which is the basis for microstructural refining in controlled rolling. Figure 14 shows an example of strain induced precipitates of niobium in a microalloyed steel with 0.02 wt% of Nb, the size of precipitates varies from 10 to 12 nm.

27 Figure 14. Strain induced precipitates of Nb observed in a carbon extraction replica obtained from a microalloyed steel [23].